1. Field of the Invention
This invention relates to a process for photoelectrochemical reduction of CO.sub.2 to hydrocarbons, predominately CH.sub.4, C.sub.2 H.sub.4 and C.sub.2 H.sub.6, in a liquid aqueous containing electrolyte dispersion of semiconductors in the presence of copper. Copper may be deposited on the semiconductor surface or dispersed in the electrolyte and transiently in contact with the semiconductor surface. The presence of copper in the photoelectrochemical reduction of carbon dioxide and/or carbon monoxide considerably increases the rate of such reduction.
2. Description of the Prior Art
Electrochemical carbon dioxide reduction using copper electrodes has been shown in a number of publications. Copper, 99.99 percent pure, was used as a cathode with 0.5M KHCO.sub.3 electrolyte for the electrochemical reduction of CO.sub.2 at ambient temperature and current density of 5.0 mA/cm.sup.2 for 30 to 60 minutes with Faradaic efficiencies for CH.sub.4 of 37 to 40 percent, Y. Hori, K. Kikuchi and S. Suzuki, "Production of CO and CH.sub.4 in Electrochemical Reduction of CO.sub.2 at Metal Electrodes in Aqueous Hydrogencarbonate Solution," Chem. Lett., 1695 (1985). In later work high purity copper cathodes, 99.999 percent, were used for the electrochemical reduction of CO.sub.2 in 0.5M KHCO.sub.3 electrolyte in a cell operated at a current of 5 mA/cm.sup.2 for 30 minutes at temperatures of 0.degree. C. and 40.degree. C. shows Faradaic efficiency for production of CH.sub.4 drops from 60 percent at 0 .degree. to 5 percent at 40.degree.; C.sub.2 H.sub.4 increases from 3 percent at 0.degree. to 18 percent at 40.degree.; while hydrogen production increases from 20 percent at 0.degree. to 45 percent at 40.degree.. It is stated that 99.99 percent pure copper cut the Faradaic efficiencies to about one-third of those obtained with 99.999 percent pure copper. Y. Hori, K. Kikuchi, A. Murata and S. Suzuki, "Production of Methane and Ethylene in Electrochemical Reduction of Carbon Dioxide at Copper Electrode in Aqueous Hydrogencarbonate Solution," Chem. Lett., 897 (1986). Later work of electrochemical reduction of CO.sub.2 at a 99.999 percent pure copper cathode in aqueous electrolytes of KCl, KClO.sub.4, and K.sub.2 SO.sub.4 at 19.degree. C. and current density of 5 mA/cm.sup.-2 showed Faradaic yields of C.sub.2 H.sub.4 of as high as in the order of 48 percent, CH.sub.4 12 percent and EtOH 21 percent. Y. Hori, A. Murata, R. Takahashi and S. Suzuki, "Enhanced Formation of Ethylene and Alcohols at Ambient Temperature and Pressure Electrochemical Reduction of Carbon Dioxide at a Copper Electrode," J. Chem. Soc., Chem. Commun., 17, 1988.
Electroreduction of CO at a 99.999 percent pure copper cathode in an aqueous catholyte of KHCO.sub.3 at ambient temperature for 30 minutes showed hydrogen to be the predominant product and at 1.0 mA/cm.sup.2 C.sub.2 H.sub.4 Faradaic production was 22 percent, CH.sub.4 1 percent; 2.5 mA/cm.sup.2 C.sub.2 H.sub.4 Faradaic production was 21 percent, CH.sub.4 16 percent and at 5.0 mA/cm.sup.2 C.sub.2 H.sub.4 Faradaic production was 16 percent, CH.sub.4 6 percent. Y. Hori, A. Murata, R. Takahashi and S. Suzuki, "Electroreduction of CO to CH.sub.4 and C.sub.2 H.sub.4 at a Copper Electrode in Aqueous Solutions at Ambient Temperature and Pressure," J. Am. Chem. Soc., 109, 5022 (1987). Similar work by the same authors showed electroreduction of CO at a 99.999 percent pure copper cathode in an aqueous 0.1M KHCO.sub.3 pH 9.6 catholyte at 19.degree. C. at 2.5 mA/cm.sup.2 resulted in Faradaic production C.sub.2 H.sub.4 of 21.2 percent; CH.sub.4 of 16.3 percent; EtOH of 10.9 percent; and 45.5 percent H.sub.2. Y. Hori, A. Murata, R. Takahashi and S. Suzuki, "Electrochemical Reduction of Carbon Monoxide to Hydrocarbons at Various Metal Electrodes in Aqueous Solution," Chem. Lett., 1665 (1987).
In the reduction of CO.sub.2 to CH.sub.4 using 99.9 percent pure cold rolled B 370 copper cathodes with a CO.sub.2 saturated 0.5M KHCO.sub.3 electrolyte, Faradaic efficiencies of 33 percent were achieved for CH.sub.4 at current densities up to 38 mA/cm.sup.2 with no C.sub.2 H.sub.4 being detected. R. L. Cook, R. C. MacDuff and A. F. Sammells, "Electrochemical Reduction of Carbon Dioxide to Methane at High Current Densities," J. Electrochem. Soc., 134, 1873 (1987). Nearly Faradaic yields of hydrocarbons by the electrochemical reduction of carbon dioxide are obtained by in situ copper deposition on electrochemical cell cathodes as described in U.S. patent application Ser. No. 234,387 entitled "Electrochemical Reduction of CO.sub.2 to CH.sub.4 and C.sub.2 H.sub.4 ", now U.S. Pat. No. 4,897,167 and as described in Ronald L. Cook, Robert C. MacDuff and Anthony F. Sammells, "Efficient High Rate Carbon Dioxide Reduction to Methane and Ethylene at in situ Electrodeposited Copper Electrode", J. Electrochem. Soc., 134, 2375 (1987).
Photoinduced carbon dioxide reduction leading to higher organic molecules has included use of specified rare earth dopants on large band gap semiconductors as described in M. Ulman, B. Aurian-Blajeni and M. Halmann, "Photoassisted Carbon Dioxide Reduction to Organic Compounds Using Rare Earth Doped Barium Titanate and Lithium Niobate as Photoactive Agents," Israel J. Chem., 22, 177 (1982). Photochemical reduction of carbon dioxide using dispersed semiconductor suspensions has been reported in a number of publications: reduction of CO.sub.2 to formaldehyde and methyl alcohol with alkaline earth titanates is taught by M. Halmann, M. Ulman and B. Aurian-Blajeni, "Photochemical Solar Collector for the Photoassisted Reduction of Aqueous Carbon Dioxide," Solar Energy, 31, 429 (1983); heterogeneous photoassisted reduction of aqueous carbon dioxide using semiconductor powders to form methanol, formaldehyde and methane with n-SrTiO.sub.3 resulting in 0.1 .mu.mole CH.sub.4 /hr is taught by B. Aurian-Blajeni, M. Halmann and J. Manassen, "Photoreduction of Carbon Dioxide and Water into Formaldehyde and Methanol on Semiconductor Materials," Solar Energy, 25, 165 (1980). Low efficiencies, 0.01 to 0.03 percent, of photoassisted reduction of CO.sub.2 over aqueous suspensions of TiO.sub.2 /SrTiO.sub.3 have been reported: A. H. A. Tinnemans, T. P. M. Koster, D. H. M. W. Thewissen and A. Mackor, "Photochemical, Photoelectrochemical and Photobiological Processes", D. O. Hall and W. Pala and D. Pirrwitz, Eds., D. Reidel Publishing Co., Dordrecht, 86, (1982). Formation of methane in photoreduction of aqueous carbon dioxide in the presence of SiC powders has been reported by D. H. M. W. Thewissen, A. H. A. Tinnemans, M. Eeuwhorst-Reinten, K. Timmer, and A. Mackor, "Photoelectrocatalytic Reactions over Aqueous Suspensions of Silicon Carbide Powders," Noveau J. de Chim., 7, 73, (1983). Photoreduction of carbon dioxide to methane in aqueous solutions with photogenerated Ru(bpz).sub.3.sup.+ in the presence of colloidal Ru has been described by Ruben Maidan, and Itamar Willner, "Photoreduction of CO.sub.2 to CH.sub.4 in Aqueous Solutions Using Visible Light", J. Am. Chem. Soc., 108, 8100 (1986). Photoreduction of CO.sub.2 to CH.sub.4 in aqueous solutions using visible light and Ru or Os colloids as catalysts is described by Itamar Willner, Ruben Maidan, Daphna Mandler, Heinz Durr, Gisela Dorr and Klaus Zengerle in "Photosensitized Reduction of CO.sub.2 to CH.sub.4 and H.sub.2 Evolution in the Presence of Ruthenium and Osmium Colloids: Strategies to Design Selectivity of Products Distribution", J. Am. Chem. Soc., 109, 6080 (1987).